1. Field of the Invention
This invention relates to novel bioactive compounds, methods of diagnostic imaging using radiolabeled compounds, and methods of making radiolabeled compounds.
2. Background Art
Monoamine neuronal systems, i.e. serotonergic, dopaminergic and adrenergic neurotransmitters, have been implicated in various neurological and psychiatric disorders. Different types of therapeutic agents aiming at these neuronal systems, as the pharmacological basis for treatment, are well known. Evaluation of the innervation of these neuronal systems is essential and important for understanding the pathophysiology, and for monitoring progress of patient treatment. New and powerful imaging methods which enable one to assess the living brain in vivo and thereby monitor the effectiveness of drugs and substances that affect brain chemistry have recently been developed. Methods such as positron emission tomography (PET) and single photon emission tomography (SPECT) involve the administration to a patient of radioactive tracer substances comprising a ligand that binds to presynaptic or postsynaptic neuroreceptors in the patient's brain. Emissions (primarily gamma rays which are emitted from the positrons or photons emitted from the radioactive tracer) are measured. These emissions are indicative of the number and degree of occupancy or blocking of the neuroreceptors. The number of neuroreceptors and the degree of occupancy or blocking is calculated utilizing a mathematical model, and compared with an intra-person or inter-person control, to determine the degree of drug response. Further treatment of the patient with drugs can be based upon the comparisons made.
The CNS neuronal systems can take up selective neurotransmitters, such as dopamine, serotonin, norepinephrine etc, from either plasma or from the synaptic cleft. This reuptake process is achieved by a selective transport mechanism based on a specific reuptake receptor on the specific type of presynaptic neuronal terminal. However, once the transmitters are inside the specific type of neuron, a second transporter or reuptake and storage mechanism is responsible for storing and packing the neurotransmitters in vesicles (or granules).
The second transport mechanism, contrary to that for the presynaptic reuptake, is based on a common ATP-dependent transporter which resides on the surface of the vesicles. The second transporters are non-selective and are effective for catecholamines, serotonin and histamine. The neurotransmitters stored in the vesicles are protected from degradation by monoamine oxidases (MAOs) in the cytosol. When neural transmissions are induced by electrical signals, the vesicles in the presynaptic neurons are fused with the membrane and the stored neurotransmitters are released into the synaptic cleft for postsynaptic receptor binding, which leads to further signal transduction.
Reserpine is a natural product which inhibits the monoamine uptake-storage mechanism of amine granules in the synapse. Tetrabenazine, 3-(2-methylpropyl)-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-benzo[a]quinolizin-2-one (TBZ), is an analog of reserpine which displays a similar biological profile. Due to their ability to deplete monoamines in the CNS, both were used as antipsychotic agents in the 1950's (Cooper J. R., Bloom F. E., Ruth R. H., In Biochemical Basis of Neurochemistry, 5th ed., Oxford University Press, New York, 1986, p. 290; Neumeyer J. L., Neuroleptics and Axiolytic Agents, In Principles of Medicinal Chemistry, Foye, W. O., ed. Lea and Febiger, Philadelphia, Pa., 1981; Kaiser C., Setler P. E., Antipsychotic Agents, Burger's Medicinal Chemistry, 4th Ed. Wolf M. E., ed. Wiley-Interscience, New York, 1981, pp 860-964). The depletion of catecholamines and serotonin in the brain by reserpine is long-lasting and the storage mechanism is irreversibly damaged. Tetrabenazine produces a similar effect; however, the drug effects of TBZ are of a shorter duration and do not cause irreversible damage to neurons (Cooper J. R., et al. In Biochemical Basis of Neurochemistry; and Neumeyer J. L. In Principles of Medicinal Chemistry). Clinical studies appear to suggest that treatment of patients with TBZ with up to 300 mg daily improved tardive dyskinesia in several trials (Neumeyer J. L.).
Recently, [3H]dihydro-TBZ (2-hydroxy-3-(2-methylpropyl)-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-benzo[a]quinolizine) has been used as a selective marker for the monoamine transport system in vitro. A detailed review of the use of [3H]dihydro-TBZ and [3H]reserpine as ligands for in vitro investigation of the monoamine transporter of chromaffin granules and CNS synaptic vesicles was published recently (Henry, J. P., Scherman D., Radioligands of the vesicular monoamine transporter and their use as markers of monoamine storage vesicles, (Commentary) Biochem. Pharmacol., 38:2395-2404, 1989). In vitro binding studies of [3H]dihydro-TBZ using membranes of chromaffin granules and brain tissue samples demonstrated a high binding affinity (Kd=2-9 nM) (Darchen F., Masuo Y., Vial M., Rostene W., Scherman D., Quantitative autoradiography of the rat brain vesicular monoamine transporter using the binding of [3H]dihydrotetrabenazine and 7-amino-8-[125I]iodoketanserin, Neurosci., 33:341-349, 1989; Meshgin-Azarian S., Chang W., Cugier D. L., Vincent M. S., Near J. A., Distribution of [3H]dihydrotetrabenazine binding in bovine striatal subsynaptic fractions: Enrichment of higher affinity binding in a synaptic vesicle fraction. J. Neurochem. 50:824-830, 1988; Near J. A., [3H]Dihydrotetrabenazine binding to bovine striatal subsynaptic vesicles, Mol. Pharmacol., 30:252-257, 1986; Scherman D., Raisman R., Ploska A., Agid Y., [3H]Dihydrotetrabenazine, a new in vitro monoaminergic probe for human brain, J. Neurochem., 50:1131-1136, 1988; Suchi R., Stem-Bach Y., Gabay T., Schuldiner S. Covalent modification of the amine transporter with N,N′-dicyclohexylcarbodiimide, Biochem., 30:6490-6494, 1991).
The regional distribution of the dihydro-TBZ binding sites in brain sections corresponded to the monoamine cell bodies and nerve endings in normal and lesioned brain sections (Masuo Y., Pelaprat D., Scherman D., Rostene W., [3H]Dihydro-tetrabenazine, a new marker for the visualization of dopaminergic denervation in the rat stratum. Neurosci. Lett., 114:45-50, 1990). Various derivatives of TBZ have been reported (Kaiser C. and Setler P. E. In Burger's Medicinal Chemistry; Neumeyer J. L., In Principles of Medicinal Chemistry; Clarke F. H., Hill R. T., Koo J., Lopano R. M., Maseda M. A., Smith M., Soled S., VonVeh G., Vlattas I., A series of hexahydro[1,4]oxazino[3,4-a]isoquinolines as potential neuroleptics, J. Med. Chem. 21:785-791, 1978; Saner A., Pletscher A., A benzo[a]quinoline derivative with a neuroleptic-like action on cerebral monoamine turnover. J. Pharmacol. Exp. Ther. 203:556-563, 1977; Lednicer D., Mitscher L. A. The Organic Chemistry of Drug Synthesis, Wiley-Interscience Inc., New York, 1977, pp 349-361; Fahrenholtz K. E., Capomaggi A., Lurie M., Goldberg M. W., Kierstead R. W. Octahydrophenanthrene analogs of tetrabenazine, J. Med. Chem. 9:304-310, 1967; Harnden M. R., Short J. H. 2-Thiol-1,3,4,6,7,11b-hexahydro-9,10-dimethoxy-2H-benzo[a]quinolizines. J. Med. Chem., 10:1183-1184, 1967; Tretter J. R., U.S. Pat. No. 3,053,845; Pletscher A., Brossi A., Gey K. F. Benzoquinoline derivatives: A new class of monoamine decreasing drugs with psychotropic action, Rev. Neurobiol., 4:275-302, 1962; Brossi A., Lidlar H., Walter M., Schnider O. 16. Synthesenversuche in der Emetin-Reihe. 1. Mitteilung. 2-Oxo-hydrobenz[a]chiolizine, Helv. Chim. Acta., 41:119-139, 1958). Reduction of the ketone to dihydro-TBZ does not affect the binding affinity. The alkylated alcohol derivatives also displayed high potency. In addition, the acetyl derivative of dihydro-TBZ has also been shown to retain high affinity for the transporter. (Scherman D., Gasnier B., Jaudon P., Henry J. P. Hydrophobicity of the tetrabenazine-binding site of the chromaffin granule monoamine transporter, Mol. Pharmacol., 33:72-77, 1988).
There are two vesicular monoamine transporters (VMAT): VMAT1, found in the adrenal tissue, and VMAT2. When located in the brain, neuronal VMAT2 is an integral part of the mechanism for the vesicular storage of monoamine neurotransmitters in brain neurons. In contrast to the situation at the synaptic membrane, where there are specific transporters for active reuptake of dopamine, serotonin or norepinephrine from the synapse, movement of monoamines (dopamine, serotonin and norepinephrine) from the cytosol to the vesicular lumen is via a common ATP-dependent transporter. Therefore, imaging VMAT2 in the brain provides a measurement reflecting the integrity (total number) of all three monoaminergic neurons (Albin R, Koeppe R., Rapid loss of striatal VMAT2 binding associated with onset of Lewy body dementia, Mov Disord., 2006:21:287-88.) Using VMAT2 as a marker of identified neuronal populations has suggested selective degeneration of projection neurons in Huntington's disease striatum (Frey K A, Koeppe R A, Kilbourn M R., Adv. Neurol.; 86:237-47; Bohnen N I, Albin R L, Koeppe R A, Wernette K A, Kilbourn M R, Minoshima S, Frey K A., J. Cereb Blood Flow Metab. (in press)).
Parkinson's disease (PD) is a movement disorder characterized by tremor and dyskinesia. Degeneration of nigrostriatal dopamine neurons plays a central role in PD. Currently, development of neuroprotective agents to slow or prevent the progression of this disease is actively being pursued. There is a compelling need for PET (positron emission tomography) and SPECT (single photon emission computer tomography) imaging agents for early diagnosis and monitoring the progression of PD (Tatsch K., Can SPET imaging of dopamine uptake sites replace PET imaging in Parkinson's disease?, For, Eur J Nucl Med Mol. Imaging., 2002:29:711-14.) On the basis of mechanisms of localization, current PET and SPECT imaging agents for PD can be generally divided into three different categories: 1. Enzymatic activity (aromatic amino acid decarboxylase, AADC); 2. Dopamine transporters (DAT); 3. Vesicular monoamine transporters (VMAT2).
The 18F labeled 6-fluoro-dopa (FDOPA) was the first PET imaging agent for PD and it remains a commonly used PET agent. It is a false substrate for aromatic amino acid decarboxylase (AADC), the first-step of synthesis of dopamine. PET imaging with [18F]6-FDOPA provides a glimpse of neuronal function—in situ synthesis of dopamine (or the lack thereof) (Brooks D J., Monitoring neuroprotection and restorative therapies in Parkinson's disease with PET, J. Neural. Transm. Suppl., 2000:60:125-37; Brooks D J., The early diagnosis of Parkinson's disease, Ann Neurol., 1998:44:S10-S18.)
The AADC is not only localized in dopamine neurons, but also in other brain cells. In the brain of PD patients the AADC enzyme is often up-regulated and the peripheral metabolites, O-methylated derivatives, will also be taken up in the brain contributing to background noise. [18F]6-FDOPA imaging reflects the loss of neuronal function related to AADC, and may underestimate the degree of neuronal loss due to compensatory changes (Tatsch K., Eur J Nucl Med Mol Imaging; Frey K A. Can SPET imaging of dopamine uptake sites replace PET imaging in Parkinson's disease? Against, Eur J Nucl Med Mol Imaging, 2002:29:715-17; Lee C S, Samii A, Sossi V, Ruth T J, Schulzer M, Holden J E, Wudel J, Pal P K, de la Fuente-Fernandez R, Calne D B, Stoessl A J., In vivo positron emission tomographic evidence for compensatory changes in presynaptic dopaminergic nerve terminals in Parkinson's disease, Ann. Neurol., 2000:47:493-503).
In the past ten years there have been a plethora of DAT imaging agents, most of which are tropane (or cocaine) derivatives which have varying degrees of affinity to serotonin and norepinephrine transporters (Meegalla S K, Plössl K, Kung M-P, Stevenson D A, Mu M, Kushner S, Liable-Sands L M, Rheingold A L, Kung H F. Specificity of diastereomers of [99mTc]TRODAT-1 as dopamine transporter imaging agents, J. Med. Chem., 1998:41:428-36; Mozley P D, Schneider J S, Acton P D, Plössl K, Stern M B, Siderowf A, Leopold N A, Li P Y, Alavi A, Kung H F, Binding of [99mTc]TRODAT-1 to dopamine transporters in patients with parkinson's disease and in healthy volunteers, J. Nucl. Med., 2000:41:584-89). A recent report pointed out the deficiencies in imaging PD based on DAT tracers, which highlighted the urgent need for imaging agents that can reliably diagnose and predict the progress of this neurodegenerative disease. (Ravina B, Eidelberg D, Ahlskog J E, Albin R L, Brooks D J, Carbon M, Dhawan V, Feigin A, Fahn S, Guttman M, Gwinn-Hardy K, McFarland H, Innis R, Katz R G, Kieburtz K, Kish S J, Lange N, Langston J W, Marek K, Morin L, Moy C, Murphy D, Oertel W H, Oliver G, Palesch Y, Powers W, Seibyl J, Sethi K D, Shults C W, Sheehy P, Stoessl A J, Holloway R., The role of radiotracer imaging in Parkinson disease, Neurology, 2005:64:208-15).
As an alternative, 11C labeled TBZ (tetrabenazine) derivatives have been successfully developed targeting VMAT2 and tested in humans (Albin R L, Koeppe R A, Bohnen N I, Nichols T E, Meyer P, Wernette K, Minoshima S, Kilbourn M R, Frey K A., Increased ventral striatal monoaminergic innervation in Tourette syndrome, Neurology, 2003:61:310-5). Animal data strongly suggested that [11C](+)-DTBZ (dihydrotetrabenazine) is less sensitive to drugs affecting dopamine levels in the brain; therefore it reflects more accurately the concentration of viable monoamine neurons. (Kilbourn M R, Frey K A, Vander Borght T, Sherman P S., Effects of dopaminergic drug treatments on in vivo radioligand binding to brain vesicular monoamine transporters, Nucl Med. Biol., 1996:23:467-71; Frey K A, Koeppe R A, Kilbourn M R. Imaging the vesicular monoamine transporter, Adv. Neurol., 2001:86:237-47; Bohnen N I, Albin R L, Koeppe R A, Wernette K A, Kilbourn M R, Minoshima S, Frey K A. Positron emission tomography of monoaminergic vesicular binding in aging and Parkinson disease, J. Cereb. Blood Flow Metab., 2006: in press; Lee C S, Schulzer M, de la Fuente-Fernandez R, Mak E, Kuramoto L, Sossi V, Ruth T J, Calne D B, Stoessl A J., Lack of regional selectivity during the progression of Parkinson disease: implications for pathogenesis, Arch. Neurol., 2004:61:1920-5). Optically resolved isomer, [11C](+)-DTBZ (labeled at the 9-MeO position), is an excellent PET tracer for measuring VMAT2 in the brain (Kilbourn M R, Lee L C, Heeg M J, Jewett D M., Absolute configuration of (+)-alpha-dihydrotetrabenazine, an active metabolite of tetrabenazine, Chirality, 1997:9:59-62; Frey K A, Koeppe R A, Kilbourn M R, Vander Borght T M, Albin R L, Gilman S, Kuhl D E., Presynaptic monoaminergic vesicles in Parkinson's disease and normal aging; Ann. Neurol. 1996:40:873-84).
Vesicular monoamine transporters (VMAT2) are also expressed in beta cells in the pancreas. The total number of binding sites for VMAT2 in the human pancreas has been determined. The Bmax=0.2 nM which translates to 12 fmol/mg of protein in beta cells. There are about 1,000,000 beta cells in human pancreas (Maffei, A, Z Liu, P Witkowski, et al. “Identification of tissue-restricted transcripts in human islets.” Endocrinology 145:4513, 2004). Insufficient beta cell mass (BCM) is a pathophysiological state of both type 1 (T1D) and type 2 (T2D) diabetes. Millions of Americans suffer from diabetes. In addition to this, many more millions have prediabetes, a condition that significantly increases the risk of developing T2D, heart disease and stroke. Diabetes is a leading cause of both acquired blindness and kidney failure in adults and is a major risk factor for both heart disease and stroke. Diabetes thus represents a major and fast growing public health burden.
Diabetes mellitus is a spectrum of disorders that all share a common abnormality of elevated blood glucose levels. Although the initial causes of this abnormality are varied (including autoimmunity, genetic risk factors, obesity, pregnancy, drugs, etc.) the common end result is a relative insulin insufficiency, i.e. the pancreatic beta cells do not produce enough insulin to meet metabolic demands (Olefsky, 2001). The two most common types of diabetes are Type I diabetes (T1D) and Type 2 diabetes (T2D).
T1D usually occurs in children or young adults and accounts for less than 10% of all cases of diabetes. T1D is caused by autoimmune destruction of beta cells leading to failure of insulin secretion. This process may take years to manifest, and during the preclinical stage autoimmune antibodies directed against beta cells can be detected in affected patients. Thus in early stages of the disease immune modulation may play an important role in treatment, while in later stages treatment will require replacement of beta cells either through regenerative or transplantation strategies.
T2D is a heterogeneous polygenic disorder that accounts for approximately 90% of all cases of diabetes. In addition to genetic risk factors, obesity, lack of physical activity and aging are important risk factors for T2D. T2D is characterized by insulin resistance, a defect which is present for years in the preclinical (prediabetes) state. This insulin resistance leads to compensatory increases in insulin production by beta cells in prediabetics. Eventually, in some patients, beta cell function then declines, leading to relative insulin insufficiency (Butler et al., 2003). Indeed, autopsy series reveal that BCM is reduced by 50-60% in T2D patients as compared to controls (reviewed in Porte and Kahn, 2001; Prentki and Nolan, 2006). This loss of beta cells may be a key step in the pathogenesis of T2D diabetes since a longitudinal study in Pima Indians suggested that beta cell failure rather than insulin resistance was the primary determinant of progression from prediabetes to diabetes (Weyer et al, 1999). Thus in T2D, insulin resistance superimposed on beta cell failure and impaired insulin secretion lead to decompensated hyperglycemia and diabetes. Disease modifying treatments for T2D must target both beta cell failure and insulin resistance in order to be most effective (Olefsky, 2001).